The present invention relates to a semiconductor device including a circuit board, a semiconductor chip on one surface of the circuit board, and a radiator base on the other surface of the circuit board for radiating heat generated in the semiconductor chip, and a method of manufacturing the semiconductor device. Specifically, the present invention relates to a semiconductor device in which the circuit board and the radiator base are bonded with soldering, and a method of manufacturing the semiconductor device.
In association with the recent improvement in performance of semiconductor devices mounted on electronic instruments, the quantity of heat generated in the semiconductor devices has increased. To dissipate the generated heat, a heat sink has been employed as external heat dissipating means. FIG. 8 is a cross sectional view of a conventional semiconductor device having such heat dissipating means.
Referring now to FIG. 8, the conventional semiconductor device 101 includes a circuit board 110; a heat generating chip (hereinafter referred to as “silicon chip” or “semiconductor chip”) 102 such as an IGBT and a FWD (free wheel diode) soldered to one surface of the circuit board 110; and a radiator base 103 soldered to the other surface of the circuit board 110 for dissipating the heat. The circuit board 110 includes an insulator substrate 111 having conductor patterns (circuit patterns) 112 and 113 made of a copper foil or an aluminum foil formed thereon. The radiator base 103 is made of a metal plate such as a copper plate.
The conductor patterns 112 and 113 are bonded to the surfaces of the insulator substrate 111 with direct bonding or active metal bonding to form the circuit board 110. The semiconductor chip 102 is soldered to the conductor pattern 112 on one of the surfaces of the circuit board 110. The radiator base 103 is soldered to the conductor pattern 113 on the other of the surfaces of the circuit board 110. Therefore, a solder layer 114 is formed between the semiconductor chip 102 and the circuit board 110, and a solder layer 115 is formed between the circuit board 110 and the radiator base 103.
As described above, the conventional semiconductor device 101 has a laminate structure of the insulator substrate (circuit board) bonded to the radiator base with soldering. In general, the insulator substrate and the radiator base are formed of materials having different coefficients of thermal expansion (CTE) as shown in Table 1. Due to the difference in the CTEs of the different materials, the radiator base 103 deforms as shown in FIG. 9 immediately after the semiconductor device or the electronic instrument having the semiconductor device is assembled.
TABLE 1InsulatorsubstrateCTERadiator baseCTEAluminum4.5 ppm/KAluminum23.1 ppm/K nitrideCopper16.5 ppm/K Alumina (Al2O3)7.8 ppm/KSilicon nitride3.4 ppm/KC/C material7.2 ppm/K
FIG. 11 is a chart showing an amount of camber Δt (μm) of a radiator base with time (h) in a case that the radiator base is bonded to the circuit board with a Pb-containing solder and in a case that the radiator base is bonded to the circuit board with a Pb-free solder. The Pb-containing solder contains tin (Sn) as a base and 60 weight % of Pb. The Pb-free solder contains tin (Sn) as a base and 5 weight % of antimony (Sb).
As shown in FIG. 11, in the case that the Pb-containing solder is used, the radiator base bonded to the circuit board returns to a flat shape with time, since the Pb-containing solder creeps and relaxes thermal stress therein. On the other hand, in the case that the Pb-free Sn solder is used in view of recent demand for environmental safety, since the solder is rigid and hardly deforms by creeping, the camber of the radiator base remains as a convex deformation. For example, when the radiator base 103 is made of a copper plate having a length of 90 mm to 110 mm, a width of 43 mm to 60 mm, and a thickness of 3 mm, the amount of the camber (Δt in FIG. 9) becomes 0.3 mm to 0.7 mm.
When such a large camber is generated, it is difficult to accurately assemble the semiconductor device in an assembling step after the soldering step. The semiconductor device 101 is provided with the radiator base 103 having the heat dissipating means 120 such as a heat sink with cooling fins. When the radiator base is cambered, a gap is formed between the semiconductor device 101 and the heat dissipating means 120, thereby reducing a contact area between the semiconductor device 101 and the heat dissipating means 120. As a result, the thermal contact resistance is increased, and it is difficult to dissipate the heat generated in the semiconductor chip 102, so that the semiconductor chip 102 may be damaged due to an abnormal increase in a temperature.
In the semiconductor device used in a vehicle, it has been required to improve reliability during a heat cycle test. A life of the semiconductor device during the heat cycle test depends on a life of the solder layer between the circuit board and the radiator base. This is because the thermal expansion difference between the soldered constituent elements (the circuit board and the radiator base) generates thermal stress in the solder layer, and finally breaks down the solder layer. Therefore, it is desirable to reduce the thermal stress and improve the thermal conduction efficiency.
Also, in the semiconductor device used in a vehicle, it has been required to make the semiconductor device light weight. To meet the demand, there has been proposed a technique in which a plate member formed of a carbon-based metal composite material is soldered to a circuit board having a circuit part mounted thereon (refer to Japanese Patent Publication (Kokai) No. 2001-58255). The carbon-based metal composite material is formed by impregnating molten aluminum, copper, silver or an alloy of these metals into a carbon pre-form made of carbon particles or carbon fibers containing graphite crystals under pressure. With the technique, the radiator base is formed of a substrate with lightweight and high thermal conductivity. The substrate also has a low modulus in a direction perpendicular to a surface thereof, so that mechanical workability thereof is excellent. Accordingly, it is possible to prevent the breakdown in the bonding portion due to the thermal stress.
Japanese Patent Publication (Kokai) No. 11-54677 has proposed a technique using a heat-dissipating carbon composite in which a liquid hardening material is impregnated into a pre-form carbon composite containing carbon fibers aligned in thickness direction, thereby improving the thermal conductivity and strength thereof.
Japanese Patent Publication (Kokai) No. 2001-39777 has disclosed a radiator base made of a short carbon fiber reinforced carbon composite having anisotropic thermal conductivity.
In the techniques described above, it is still difficult to sufficiently improve the heat dissipation performance and prevent the deformation in the bonding portion due to the thermal stress, thereby making it difficult to obtain a semiconductor device exhibiting excellent performance.
In view of the problems described above, an object of the present invention is to provide a semiconductor device in which the heat dissipation performance thereof is improved, and the deformation in the bonding portion thereof due to the thermal stress is prevented. Further, it is possible to accurately assemble the semiconductor device in the manufacturing process, thereby improving the reliability and reducing a weight thereof.
Another object of the present invention is to provide a method of manufacturing the semiconductor device.
Further objects and advantages of the invention will be apparent from the following description of the invention.